1. Field of the Invention
The present invention relates to an apparatus for applying a liquid or viscid coating medium onto a moving material web, especially one made of paper or cardboard.
2. Description of the Related Art
An exemplification of a known apparatus for applying a liquid or viscid coating medium onto a moving material web is illustrated in FIG. 2a. The coating apparatus includes an applicator roll 12 and a counter roll 14. The two rolls, 12 and 14, form a nip through which material web 16 runs in the direction of feed. Applicator roll 12 is coated with a liquid or viscid coating medium by use of a coating mechanism 18. This medium is subsequently transferred onto the material web 16 in the area of the coating location S. Applicator roll 12 and counter roll 14 are driven in opposite directions around their respective axes A and B as indicated by arrows P and Pxe2x80x2. Such coating apparatuses are applied, for example, in film coating processes, roll coating processes and size presses.
Coating apparatus 10 usually experiences problems in the area Spxe2x80x94located adjacent to the coating location S, on the downstream side relative to the movement of the material web 16xe2x80x94as the material web 16 separates from the outer surface 12a of roll 12. The pre-metered coating of coating medium 20 being applied onto the outer surface 12a is only partially transferred in the nip onto the material web 16. A part 20b of the coating medium 20 adheres to the outer surface of roll 12, and therefore, returns back to the coating mechanism 18. Those experienced in the art, refer to this effect as xe2x80x9cfilm splittingxe2x80x9d.
In the process of film splitting, xe2x80x9ccoating medium threadsxe2x80x9d F (FIG. 2b) develop between the coating medium 20 being applied onto the material web 16 and the remaining layer 20b of coating medium on roll 12xe2x80x94similar to the honey threads that are formed between a spoon that is pulled from a honey jar. When these threads F are torn, it is possible to develop droplets or particles 20c of the coating medium 20, which is reflected in the formation of spray mist N. Accordingly, it can be said that the spray mist is formed by the particles and droplets 20c of the coating medium 20, which, for example, are torn from the previously applied coating 20a by the adhesive force of applicator roll 12.
Due to the aforementioned formation of the spray mist, the quality of the coated material web can be compromised in several ways. For example, small craters can form on the outer surface of the coated material web. Furthermore, the remaining parts of the torn-off coating medium threads F can cause a so-called xe2x80x9corange skin effectxe2x80x9d. Additionally, the re-deposit of droplets or particles 20c in an area removed from the spray area Sp can lead to an irregular structure of the material web""s coating surface. The spray mist also causes other drawbacks such as a contamination of the coating apparatus.
The risk of encountering the above-stated disadvantages increases as the feed speed of the material web 16 increases, as the area specific coating weight on the material web 16 increases, and as the absorbency capability and/or the bibulousness of the material web 16 decreases. To avoid the aforementioned disadvantages of the state of the art, it was suggested in a summary report issued by the xe2x80x9cTappi Metered Size Press Forumxe2x80x9d by Dr. Hans W. Maurer (weekly publication for paper manufacturing 17, 1997, page 827) to vary the composition of the coating medium. This approach, however, has proven to be impractical.
The present invention provides a coating apparatus of the type mentioned above, which eliminates the spray mist from the coating medium, or, at a minimum, reduces its adverse effects on the coating result, so that a uniform coating can be achieved.
A coating mechanism first applies the medium onto an applicator element or an applicator roll, which subsequently transfers the coating medium at a coating location onto the material web in the form of a coating layer, or it is entered into a coating sump, formed between the material web and a limiting element or limiting roll in which the coating medium is deposited onto the material web. If so desired, a doctoring device can be positioned at a doctor station, adjacent to the coating location, on the downstream side relative to the movement of the material web, for the purpose of smoothing and/or metering the coating.
This problem is solved by providing an apparatus to generate and/or supply fluid, preferably steam. The apparatus is positioned adjacent to the coating location or doctor station on the downstream side relative to the movement of the material web.
For the sake of simplicity, further elaboration of this invention will be conducted by using steam as the fluid that is being introduced in the area of the coating location. It is, however, understood that other fluids can also be successfully used, not only in the area of the coating location, but also in the area of a doctor station at which the coating is smoothed and/or metered by use of a roll doctor, which also has the tendency to develop spray mist.
In accordance to this invention, a device provided to surround the spray area Sp, located adjacent to coating location S on the downstream side relative to the movement L of the material web, with a steam-saturated atmosphere, preferably steam produced from water. This causes, on one hand, a reduction in the surface tension of the coating medium, which tends to favorably affect the thread and droplet formation. On the other hand, the steam-saturated atmosphere causes a slowing of the drying process of the coating medium, as well as the medium droplets that are being applied onto the material web, so that the structures on the coating that are being produced by tearing threads and droplets can bleed into the coating without leaving any marks. This results in a more uniform coating.
It has been proven to be advantageous to use steam that no longer absorbs any water, i.e., saturated and superheated steam.
Regardless of whether the steam is produced near the coating apparatus or whether the steam is produced in a remote area relative to the coating apparatus and subsequently piped to the spray area, it is desirable for the steam-generating apparatus and/or the steam supply apparatus to point the steam directionally towards the coating location.
For example, the steam-generating apparatus and/or the steam supply apparatus can include a steam supply line in the area of the coating apparatus. The line has at least one steam discharge opening facing the coating location.
In accordance to a first design variation, the aforementioned advantages provided by this invention can be realized with a steam-saturated atmosphere which is, by and large, stationary in the spray area, if one ignores the effects of the material web movement, the steam supply effects and any residual spray mist effects.
According to an alternative design variation, it is, however, also possible to obtain these same advantages with a dynamic steam-saturated atmosphere, which is achieved by blowing the steam into the spray area, past the steam-generating apparatus and/or the steam supply apparatus, and then exiting past the spray area. For that purpose, the steam discharge opening that forms the wall sections of the steam supply line, or the wall sections downstream of the steam discharge opening, can be formed nozzle-like, preferably with the smaller nozzle area on the downstream side.
The movement of the steam that is discharged can be affected by appropriate shaping of the outer surfaces of the steam-generating apparatus and/or the steam supply apparatus in such a way that a further smoothing of the coating on the material web can be achieved. For example, the steam-generating apparatus and/or the steam supply apparatus can be designed as a flow restrictor unit in such a way that the steam-saturated atmosphere, which is being discharged in the area adjacent to the coating location, is done so at a fluid velocity that forces at least part of the droplets having formed in the approximate area of the coating location back into the coating. Since the droplets, as well as the coating, are still sufficiently moist in the described area as a result of the steam-saturated atmosphere, the droplets, which are forced back, bleed into the coating without leaving any marks.
In order to guard against any contamination of the coating apparatus as a result of any residual spray mist remaining in the area in spite of the steam-generating apparatus and/or the steam supply apparatus (designed as described herein, a collection device is positioned on the side of the steam-generating apparatus and/or the steam supply apparatus that is facing away from the coating location. This device collects those coating medium droplets and particles which have not been returned to the material web, and/or collects those coating medium droplets and particles that are not adhering to the coating.
Although the droplets of the residual spray mist do not dry as quickly because of the steam-saturated atmosphere, it is appropriate to guard against a xe2x80x9cbakingxe2x80x9d of the coating medium on the splash panel of the collection device. To this end, it is suggested to design a splash panel that includes cooling fins and/or a cooling coil and/or a heat exchanger. Additionally, or alternatively, this splash panel can be equipped with a device to supply a rinsing agent. The collected coating medium and/or the rinsing agent can be removed from the collection device via a discharge line. It is further possible to position a suction apparatus at the side of the steam-generating apparatus and/or the steam supply apparatus that is facing away from the coating location in order to evacuate the steam-saturated atmosphere and/or those coating medium droplets and particles which have not been returned to the material web, and/or to collect those coating medium droplets and particles that are not adhering to the coating. In doing so, the suction apparatus and the aforementioned collection device should be designed as one functional unit.
The steam-generating apparatus and/or the steam supply apparatus described in this application can be applied to an apparatus designed for a one-sided application of the coating medium onto the material web, or it can be applied to an apparatus designed for a two-sided application. In the case of the two-sided application of the coating medium onto the material web, each side of the material web is equipped with a steam-generating apparatus and/or the steam supply apparatus.
In order to combat the adverse effects of film splitting on especially susceptible sections of the material web with a more intense steam treatment, a plurality of steam-generating and/or steam supply sections are positioned in a sequential manner and perpendicular to the material web. These units are capable of being controlled independently from one another, at least in terms of the amount of steam per unit time that is being discharged.
This invention further relates to a process for the application of a liquid or viscid coating medium onto a moving material web, especially one made of paper or cardboard. With respect to the advantages that can be achieved with this coating process, the aforementioned discussion of the coating apparatus applies.